IsAssignableFrom or AS?

Question

I have next code:

private T CreateInstance<T>(object obj) // where T : ISomeInterface, class
{
    ...

    if (!typeof(T).IsAssignableFrom(obj.GetType())) { throw ..; }

    return (T)obj;
}

Can it be replaced with this:

T result = obj as T;

if (result == null) { throw ..; }

return result;

If not - why?

Answer 1

What about if (!(bar is T)) { throw ..; }

Alternatively if you don't need your own exception message the simplest answer is just to do:

return (T)obj;

The reason if that if it's not castable an InvalidCastException will be thrown and the return ignored. Unless you're adding some more logic or a custom error message there's no need to do a check and throw your own exception.

Answer 2

Another variant:

private T CreateInstance<T>(object obj) where T : ISomeInterface // as OP mentioned above
{
    ...

    T result = obj as T;
    if (result == null)
        { throw ..; }
    else 
       return result;
}